Trojan Source

Trojan Source is the name of a software vulnerability that abuses Unicode's bidirectional characters to display source code differently than the actual execution of the source code.^[1] The exploit utilizes how writing scripts of different reading directions are displayed and encoded on computers. It was discovered by Nicholas Boucher and Ross Anderson at Cambridge University in late 2021.^[2]

Background

Unicode is an encoding standard for representing text, symbols, and glyphs. Unicode is the most dominant encoding on computers, used in over 98% of websites as of September 2023^[update].

In the exploit, bidirectional characters are abused to visually reorder text in source code so that later execution occurs in a different order. Bidirectional characters can be inserted in areas of source code where string literals are allowed. This often applies to documentation, variables, or comments.

def sum(num1, num2):
  '''Add num1 and num2, and [RLI] return; '''
  return num1 + num2

def sum(num1, num2):
  '''Add num1 and num2, and return; '''
  return num1 + num2

def sum(num1, num2):
  '''Add num1 and num2, and ''' ;
  return
  return num1 + num2

In the above example, the RLI mark (right-to-left isolate) forces the following text to be interpreted in the reverse order: the triple-quote is first (ending the string), followed by a semicolon (starting a new line), and finally with the premature return (returning None and ignoring any code below it). The new line terminates the RLI mark, preventing it from flowing into the below code. Because of the Bidi character, some source code editors and IDEs rearrange the code for display without any visual indication that the code has been rearranged, so a human code reviewer would not normally detect them. However, when the code is inserted into a compiler, the compiler may ignore the Bidi character and process the characters in a different order than visually displayed. When the compiler is finished, it could potentially execute code that visually appeared to be non-executable.^[5] Formatting marks can be combined multiple times to create complex attacks.^[6]

Impact and mitigation

Programming languages that support Unicode strings and follow Unicode's Bidi algorithm are vulnerable to the exploit. This includes languages like Java, Go, C, C++, C#, Python, and JavaScript.^[7]

While the attack is not strictly an error, many compilers, interpreters, and websites added warnings or mitigations for the exploit. Both

Red Hat issued an advisory on their website, labeling the exploit as "moderate".^[12] GitHub released a warning on their blog, as well as updating the website to show a dialog box when Bidi characters are detected in a repository's code.^[13]

References

External links

• https://trojansource.codes/ site by the discoverers, Nicholas Boucher and Ross Anderson
  • Proof of concept code
  • Trojan Source full research paper
• NIST National Vulnerability Database & CVE Common Vulnerabilities and Exposures
  • CVE-2021-42574 - NIST & CVE (BIDI exploit)
  • CVE-2021-42694 - NIST & CVE (homoglyph attack)
• UAX 9 from the Unicode Consortium about bidirectional characters and formatting
  • Unicode UTR 36 from the Unicode Consortium, which describes the vulnerability in Unicode
• CERT/CC vulnerability report